Optocoupler for the control of high voltage
The present invention relates to an optocoupler including a light source having a body and electrical leads, a light detector having a diode stack a metal end cap and electrical leads, and an optical cavity including optically transparent material at least partially covering the body of the light source and the diode stack of the light detector. Also included is a reflective layer including optically reflective material surrounding the optical cavity. The electrical leads of the light source, the metal end cap and the electrical leads of the light detector protrude from the optical cavity and the reflective layer.
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CROSS REFERENCE TO RELATED APPLICATION
This application is a U.S. Divisional Application which claims priority to U.S. patent application Ser. No. 15/660,008, filed Jul. 26, 2017, the contents of such application being incorporated by reference herein.
The present invention relates to an optocoupler for the control of high voltage.
Performance (e.g. efficiency) of an optocoupler is typically evaluated based on current transfer ratio (CTR). CTR is essentially a ratio of output current to input current expressed as a percentage. It is well known that as the voltage rating of the optocoupler increases, CTR decreases. State-of-the-art optocouplers are able to achieve respectable CTRs at low voltages. However, once rated operating voltage reaches a certain level (e.g. above 10 KV), the CTR of these state-of-the-art optocouplers quickly drops to an unacceptable level (e.g. CTR<0.5%). This is attributed to inefficient optical coupling and packaging of these devices.
An embodiment includes an optocoupler. The optocoupler includes a light source having a body and electrical leads, a light detector having a diode stack a metal end cap and electrical leads, and an optical cavity including optically transparent material at least partially covering the body of the light source and the diode stack of the light detector. Also included is a reflective layer including optically reflective material surrounding the optical cavity. The electrical leads of the light source, the metal end cap and the electrical leads of the light detector protrude from the optical cavity and the reflective layer.
Another embodiment includes a method of manufacturing an optocoupler. The method includes positioning a body of a light source into a first mold such that electrical leads of the light source protrude from the first mold, positioning a diode stack of a light detector into the first mold such that metal end caps and electrical leads of the light detector protrude from the first mold, pouring optically transparent material into the first mold to create an optical cavity enclosing the body of the light source and the diode stack of the light detector, and disposing a coating of optically reflective material over the optical cavity to form a reflective layer.
BRIEF DESCRIPTION OF THE FIGURES
Aspects of the present invention provide an optocoupler designed and manufactured for efficient use in high voltage applications in which the optocoupler may have several hundred to many thousands of volts across its output terminals. The optocoupler is designed and manufactured to increase optical efficiency when transferring light from a light source to a light detector, thereby maintaining an adequate CTR even at higher rated operating voltages (e.g. >10 KV).
High voltage applications are common in various industries. Once such application is in mass spectrometry (i.e. the determination of mass of a sample using a mass spectrometer). In general, a mass spectrometer 100 (see
For low voltage control circuit 102 to safely control high voltage circuit 106, without damaging the low voltage components, optocoupler 104 is employed as a link between the circuits. Optocoupler 104, through an optical interaction between a light source (not shown) and a light detector (not shown), electrically isolates low voltage control circuit 102 from high voltage circuit 106, while allowing control over the high voltage (e.g. switching electrical currents on/off).
For example, during operation of the mass spectrometer, a processor (not shown) in low voltage control circuit 102, is able to control the high voltage devices (e.g. electron gun) in high voltage circuit 106, by sending control signals to optocoupler 104. These control signals are converted by the light source in optocoupler 104 into light beams. The light detector in optocoupler 104 then receives these light beams. Upon receiving the light beams, the light detector modulates the amplitude of electrical current flowing through optocoupler 104 to the high voltage circuit 106 (e.g. the light detector may be configured as a binary switch to allow or prevent current from flowing through the high voltage circuit based upon its detection or non-detection of light, or it may regulate the amount of current flowing through the high-voltage circuit based upon the amount of light detected).
One or more light sources and one or more light detectors within optocoupler 104 may be arranged in various configurations to perform optical coupling. Two examples of these configurations are shown in
Typically, the photo-diodes are housed together in a single package.
The diode stack of
Once the number, position, and configuration of light sources and light detectors is decided, these components are enclosed together in an optical cavity to create the optocoupler for installation in the end device (e.g. mass spectrometer). For example, as shown in
Another view of optocoupler 600 is shown in the schematic drawing of
For example, the components may be positioned relative to each other in a mold which is then filled with a material that is optically clear for the wavelength of the LED, such as but not limited to, for example, epoxy, plastics, acrylic, glass or silicon. The optical material provides a rigid structure for forming the optocoupler body, as well as an optical medium for transferring light between the LEDs and photo-diodes. This provides a configuration to securely encapsulate the components into a single device that may be used in various applications such as in the mass spectrometer. It should be noted that in this configuration, the electrical leads of the LEDs and the electrical leads (not shown) of the photo-diodes protrude from optical cavity 700 allowing for electrical connections to the low voltage and high voltage circuits respectively.
Positioning of the LEDs relative to the photo-diodes is performed, at least in part, based on the optocoupler rated operating voltage. As discussed above, operating voltages may be high (e.g. >10 KV) for certain applications. Higher rated operating voltages require LEDs to be distanced further from the photo-diodes to prevent the high voltage from jumping from the high voltage circuit to the low voltage circuit. This distance may be determined based on the optical cavity 700 material dielectric breakdown strength (i.e. the minimum applied voltage divided by electrode separation distance that results in breakdown). Specifically, for a given design operating voltage of the optocoupler (such as a maximum design operating voltage, or a maximum design operating voltage or normal operating voltage multiplied by a predetermined safety factor), the distance is chosen to be greater than the distance corresponding to the material's dielectric breakdown strength.
In addition, positioning of the LEDs relative to the photo-diodes is also performed, at least in part, based on obtaining a desired optical efficiency. In general, optical efficiency decreases with an increase in distance between the LEDs and the photo-diodes (e.g. the farther away the components are, the less efficient the optocoupler becomes). This is one reason why the optocouplers with a higher operating voltage have lower CTRs. Thus, it is beneficial to set the distance between the LEDs and the photo-diodes to be no larger than necessary (e.g. minimum distance) to protect the low voltage circuit from the high voltage jumps.
As can be seen in
Hot spots can be problematic, because the photo-diode's ability to conduct electrical current through the high voltage circuit is proportional to the amount of light impinging on them. Thus, the photo-diodes (e.g. 402, 404, 410 and 412) that are directly in front of the LED may be fully conductive, whereas the photo-diodes (e.g. 406 and 408) that are not directly in front of the LEDs receive less light are only partially conductive.
Problems associated with hot spots may be further exacerbated by the fact that the photo-diodes are electrically connected in series, and the total amount of electrical current able to flow through a series connection is limited to the least conductive of the photo-diodes in the stack. For example, if some (e.g. 402, 404, 410 and 412) of the photo-diodes receives direct light, it may become fully conductive having the capability to conduct ‘C’ amps of current. However, if the other photo-diodes (e.g. 406 and 408) are only receiving indirect light, then it may only by half conductive having the capability to conduct C/2 amps of current. Due to the series connection between the three photo-diodes, the maximum current that can flow through the optocoupler will be C/2 amps, which may be insufficient for the given application.
One solution to this hot spot problem is the use of a reflective layer for improving what is herein referred to axial uniformity (e.g. the uniformity of light received across all the photo-diodes in the stack). As described above, the LEDs and photo-diodes are molded using a clear material. This provides a medium for light to travel between the LEDs and the photo-diodes. To ensure that all of the photo-diodes (e.g. even the diodes indirectly positioned relative to the LEDs) receive a similar amount of light, the optocoupler is coated with a reflective material having a high level of reflectance (e.g. >90%).
For example, the molded optical cavity 700 (once cured) may be inserted into a second mold which is then filled with a reflective material. This reflective material encapsulates the optical cavity and provides a surface that reflects the light beams emitted by the LEDs back to the photo-diodes. The reflective material may allow both the specular reflection (e.g. light beam is directly reflected in a single direction) and diffuse reflection (e.g. light beam is diffused in many directions) of the light beams.
For example, assume that cavity 700 of the optocoupler in
As described above, the reflective layer ensures that light beams attempting to exit the optical cavity are reflected back into the optical cavity towards those photo-diodes that may not receive direct light from the LEDs. This configuration is beneficial in avoiding the so called hot spots (e.g. conditions in which some photo-diodes don't receive enough light).
As shown in data plot 802, were the LEDs are oriented directly at the detector, the irradiance is not uniform. The reflective layer sees a small portion of the light from the LEDs and scatters it back to the detector while most of the light from the LEDs directly impinges on the detector. For example, the photo-diode positioned at axial position 0 (e.g. positioned indirectly with respect to the LEDs) receives significantly less light than the photo-diodes positioned at axial positions −2 and 2 (e.g. positioned directly in front of the LEDs). This is an indication of hot spots due to non-uniform distribution of light.
In contrast, as shown in the data plot 806, where the LEDs point indirectly at the light detector, the irradiance becomes more uniform. For example, the photo-diode positioned at axial position 0 (e.g. positioned indirectly with respect to the LEDs) receives almost the same amount of light as the photo-diodes positioned at axial positions −2 and 2 (e.g. positioned directly with respect to the LEDs). This is due to larger amount of light scattering off the reflective layer.
In many cases, it may not be acceptable to mount the light sources at oblique angles as shown in 804. Instead, the shape of the optical cavity in conjunction with a reflective layer, can improve the optocoupler's optical efficiency. An example of an optical cavity that accomplishes this is shown in
In one example, optocoupler 900 is shown in
In the embodiment depicted in
The width of the optical cavity as shown in
Optocoupler 1100 in
In one example, the optical cavities (e.g. 1102) represent volumes formed in a molding process, where the photo-diode stack and LEDs may be installed after the mold is cured. In another example, these cavities represent volumes in the optocoupler where the photo-diode stack and LEDs are molded directly into the optical cavity such that active regions (light receiving diode stack surfaces and LED light emitting surfaces) are enclosed in optical cavity 1102, while inactive regions (electrical leads and end caps) are not enclosed in optical cavity 1102 (e.g. they protrude from optical cavity 1102).
In general, optocoupler 1100 is designed to increase efficiency by one or more of the following: 1) limiting the overall optical cavity size to only include the active photo-diode stack regions and the active light emitting LED surfaces (e.g. electrical leads and end caps are not in the optical cavity) 2) covering the optical cavity in a coating that promotes both reflected and scattered light, 3) using a geometry that is conducive to minimizing a number of reflections required by a beam of light before impinging on the photodiode stack, and 4) positioning the LEDs with respect to the diodes stack to maximize axial uniformity over the stack.
The types of LEDs used in these optocouplers may vary in size and shape based on the LED design, and may include LEDs that emit light in various spectra (visible spectrum, infrared spectrum, etc.). An example of a compatible LED for installation in the optocouplers (e.g. optical cavity 1000 in
The above described LED configuration allows the LED to be inserted or molded into the optical cavity. An example of the LED inserted or molded into optical cavity 1100 is shown in
As described above, the optocoupler includes light sources and a light detector mounted or molded in an optically clear material at the wavelength of the light sources to form an optical cavity. To improve optical efficiency, the optical cavity is encapsulated in a reflective layer. An example of an optocoupler having these two layers is shown in
Optocoupler 1400 shown in
In a first molding step 1506, a clear material such as a clear epoxy is poured into the mold and then oven cured in step 1508. Once the epoxy has cured, an electrical test is performed on the LEDs and photo-diodes in step 1510. If the LEDs or photo-diodes fail the test, then the device is scrapped in step 1512. However, if the LEDs and photo-diodes pass the test, then a second molding process is performed in step 1514.
Specifically, the cured epoxy extracted from the first mold is inserted into a second mold which also allows the electrical leads 1420-1426 of the LEDs and photo-diodes to protrude from the mold. A reflective material, such as but not limited to a mixture of epoxy and titanium dioxide is then poured into the second mold to encapsulate the cured clear material (e.g. silicon) and allowed to oven cure in step 1516. Once this second oven-curing procedure is complete, the LEDs and photo-diodes are once again electrically tested in step 1518. If the LEDs or photo-diodes fail the test, the device is scrapped in step 1520. However, if the LEDs and photo-diodes pass the test, the device is deemed to be good product in step 1522 and is packaged for sale to consumers.
In an alternative manufacturing process, the LEDs and photo diodes may be mounted in the optocoupler after the molding process. For example, the first mold may include a shape that creates cavities for installation of the LEDs and photo-diodes. The optocoupler may be molded with the clear epoxy, and then the reflective epoxy in a two-step process similar to that described in
The optocoupler design increases the CTR for a given operating voltage by optimizing the optical efficiency and axial uniformity of the optical power. Optical efficiency is the ratio of the optical power hitting the light detector (i.e. diode-stack) versus the optical power emitted by all the light sources (i.e. LEDs). The axial uniformity of light received by the detectors is computed as (min_intensity/max_intensity)*100), where max_intensity is the maximum intensity of light received by one of the diodes (e.g. diode receiving direct light) in the stack, and min_intensity is the minimum intensity of light received by one of the diodes (e.g. diode receiving reflected light) in the stack. For example, the optocoupler may be designed to achieve an optical efficiency of EFF>50% for a given input of 140 mW of optical power, and a uniformity U>85%.
To achieve the desired CTR, the optocoupler design incorporates a number of design factors. A first factor is the optical cavity volume. For example, the design may minimize the optical cavity volume such that light has no paths that do not lead to the detector either directly or through a single reflection. A second factor is the placement of the LED with respect to the detector which maximizes the axial uniformity of the light hitting the detector by using a combination of direct and indirect (single reflection) lighting. A third factor is ensuring the high reflectance surface is both highly reflective (e.g. preferably >80% reflectivity, or more preferably at least 90%), and causes diffusion so that it scatters the light in an even pattern. A fourth factor is to exclude or minimize inactive regions (e.g. metal end caps, leads, LED body, etc.) from the optical cavity.
In addition, an encapsulant (e.g. epoxy, plastics, silicon, ceramic) may be used to maintain adequate voltage isolation between the electronic components (e.g. leads). LEDs with an emission spectra wavelength, matched to the peak detector detection wavelength should be used. The encapsulant can also act as the reflective material and does not need to be applied in a separate operation.
Although manufacturing includes a second molding process for adding the reflective layer (as shown in
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather various modifications may be made in the details within the scope and range of equivalence of the claims and without departing from the invention.
1. A method of manufacturing an optocoupler comprising:
- positioning a body of a light source into a first mold such that electrical leads of the light source protrude from the first mold;
- positioning a diode stack of a light detector into the first mold such that metal end caps and electrical leads of the light detector protrude from the first mold;
- pouring optically transparent material into the first mold to create an optical cavity enclosing the body of the light source and the diode stack of the light detector; and
- disposing a coating of optically reflective material over the optical cavity to form a reflective layer.
2. The method of claim 1, wherein the step of disposing the coating of optically reflective material over the optical cavity further comprises the steps of:
- placing the optical cavity into a second mold such that the electrical leads of the light source and the metal end caps and electrical leads of the light detector protrude from the second mold;
- pouring the optically reflective material into the second mold to create the reflective layer, wherein the reflective layer fully covers the optical cavity; and
- extracting the optocoupler from the second mold.
3. The method of claim 1, further comprising:
- positioning the body of the light source and the diode stack of the light detector a distance away from each other in the optical cavity before pouring the optically transparent material into the first mold, the distance selected to be greater than a distance corresponding to a dielectric breakdown strength of the optically transparent material at a design operating voltage of the optocoupler.
4. The method of claim 1, wherein the light detector comprises a plurality of photo-diodes, the method further comprising:
- providing the diode stack about an axis that intersects the optical cavity before pouring the optically transparent material into the first mold.
5. The method of claim 1, further comprising:
- selecting a shape for the optical cavity that achieves a predetermined uniformity and a predetermined efficiency of light received by the light detector.
6. The method of claim 1, further comprising:
- minimizing a volume of the first mold by spacing the light source and the light detector based on a design operating voltage of the optocoupler, dielectric breakdown strength of the optically transparent material, and uniformity of light received by the light detector.
Filed: Sep 21, 2018
Date of Patent: Jul 2, 2019
Patent Publication Number: 20190044020
Assignee: HARRIS CORPORATION (Melbourne, FL)
Inventors: Stuart D. Wood (Macedon, NY), Steven M. DeSmitt (Fairport, NY), Eugene G. Olczak (Pittsford, NY)
Primary Examiner: Cuong Q Nguyen
Application Number: 16/137,760
International Classification: H01L 31/18 (20060101); H01L 31/147 (20060101); H01L 31/0232 (20140101); H01L 31/02 (20060101); H01L 31/0203 (20140101);